Development of a Near-Infrared Photoacoustic System for Selective, Fast, and Fully Automatized Detection of Isotopically Labeled Ammonia

Different environmental and industrial technologies seek for fast and automatic ammonia detection systems, capable of the selective measurement of the concentration of its isotopes at sub-ppm levels, without any interference with the common contaminants. In this work, we report the quasi-simultaneous measurement of 14NH3 and 15NH3 concentrations based on a near-infrared diode laser-based photoacoustic system. Using a widely tunable external cavity diode laser, four nearby wavelengths within the range of 1531.3–1531.8 nm were optimal circumstances for sensitive detection, while avoiding interference with water vapor. Subsequently, a more robust distributed feedback diode laser was employed to tune the laser wavelength on the sub-second timescale by varying its driving current rather than using much slower temperature tuning. The detection limit of our system is 0.15 and 0.73 ppm for 14NH3 and 15NH3 (with an accuracy below 0.1%), respectively, and the response time is 3.5 s.


INTRODUCTION
Industrial ammonia (NH 3 ) production through the Haber− Bosch process is a significant contributor to climate change, accounting for 1.2% of the global CO 2 emissions. It has therefore become imperative in the scientific community to develop alternative methods for NH 3 synthesis. Electrochemical nitrogen (N 2 ) conversion to NH 3 has become a popular research field during the past decade, 1 as it holds the promise of substituting the energy-consuming and polluting Haber−Bosch process. Because of the infancy of this field, the NH 3 generation rates are generally very small (often resulting in concentrations well below 1 ppm) to an extent that the detected amounts can easily originate from different contamination sources (air, human breath, N 2 gas source, unstable N-containing compounds, etc.). In a recent article, rigorous isotopic labeling measurements were integrated in N 2 −to−NH 3 conversion studies. 2 Rather shockingly, it was found that the metallic catalysts, which are the most active in aqueous solutions, did not generate any NH 3 . This observation highlights that sensitive detection of NH 3 is not enough, and one also needs to be able to detect 15 NH 3, in the presence of 14 NH 3 and N 2 gases. Such a protocol can prevent false-positive results, while also providing information on the presence of contaminants. Furthermore, to perform mechanistic studies with appropriate time resolution, we need analytical methods, which can be connected in-line and can provide (quasi)-realtime information on the product formation (i.e., separation or derivative formation-based methods are excluded). In addition, isotopologues of NH 3 ( 14 NH 3 and 15 NH 3 ) are commonly applied in physiology, for metabolic tracing studies where they help the identification of biosynthetic pathways used by cells. 3 Another major application is in environmental monitoring (e.g., water and air quality and exhaust gas analysis) and in industrial process control (e.g., chemical, pharmaceutical, and propulsion).
Although there is a wealth of analytical techniques and methods for NH 3 detection, their selectivity, sensitivity, and response time with an appropriate detection limit have been an important analytical problem for decades. Starting from wet chemical methods, through physical and chemical interactionbased ones, a wide sort of spectroscopic methods (spanning through the whole electromagnetic spectrum) have been employed (Table 1). As shown in Table 1, many of the physical and chemical methods are applicable for the bulk (nonselective) detection of 14 NH 3 and 15 NH 3 isotopologues.
The pool narrows drastically when an isotopic analysis is also required. Such methods are mostly based on mass spectrometry (MS), such as isotope ratio mass spectrometry for simultaneous detections of 14 NH 3 and 15 NH 3 isotopes. This method has a number of disadvantages; first of all, it involves complicated sample preparations (aqueous phase samples are analyzed), which is in many cases a source of artifact or other possible bias and error during measurements. Moreover, it is frequently combined by headspace, elemental analyzer, or ion chromatograph. The use of MS-based methods is challenging in general, because the mass difference between H 2 O and 15 NH 3 is very small (0.008 AMU); therefore, the suppression of the water signal becomes necessary to quantify the 15 NH 3 signal. This often yields to uncertainties in the detection − especially at low ammonia concentrations. 49 Fourier transform infrared detection is based on the change in the vibrational frequencies because of the different molar masses. While this method is cheaper, easier to operate, and faster, compared to MS-based methods, it has disadvantages of requiring a considerably high amount of gas sample, low selectivity, and limited system stability. 20−23 NMR spectroscopy is also an option, as both the 15 NH 3 signal and the splitting of the 1 H resonance (because of the 1 H and 15 NH 3 / 14 NH 3 interactions) can be traced. Unfortunately, both options are hindered at low concentrations and require relatively long measurement times. Overall, according to the best of our knowledge, there is no method that fulfills all the requirements listed in the first paragraph. Interestingly, most of the precedent literature specializes in the measurement of nitrogen isotope from either NH 4 + ion or NH 3 gas using various adaptations of the ammonia diffusion method. 50 The reasons for this could be the low concentration of NH 3 (in the sub-μg/L range), which presents a detection challenge because of the poor sensitivity of the currently used methods, mostly based on compoundspecific isotope analysis (CSIA). 51,52 Another possible measurement technique is the PA detection that in principle allows the in situ and nondestructive measurement of isotopes. PA spectroscopy is a powerful technique to measure concentrations at the low levels (from ppm to ppt ranges, depending on the available light source). 4,24−28 In a PA detector, acoustic pressure waves recorded by a microphone are generated because of molecular absorption of modulated optical radiation in gases, liquids, and solids. In case of gases, the amplitude of the generated sound is directly proportional to the concentration of the absorbed gas component.
This work aims to develop a relatively simple, yet reliable, fully automatic, and robust system for the selective, rapid, and sensitive measurement of ammonia isotopes by using a nearinfrared photoacoustic (NIR-PA) system. The PA method is efficient in determining NH 3 concentration in general, and to the best of our knowledge, there is only one report on measuring 14 NH 3 and 15 NH 3 selectively. 4 In this study, a procedure is used where the optical path length is of the order of 10 m through a generated plume making its application in laboratory impossible. Here, we describe a newly developed PA method for the simultaneous detection of 14 NH 3 and 15 NH 3 isotopologues, with a small and easy-to-use device.

EXPERIMENTAL SECTION
Spectral measurements, calibrations, cross-sensitivity determinations, and response time measurements were executed using the gas generation system shown in Figure S1. It has two main parts: the ammonia gas generation unit that generates various mixtures of 14 NH 3 and 15 NH 3 and the NIR-PA system, operated either by an external cavity diode laser (ECDL) or by a distributed feedback (DFB) diode laser. The gas generator unit was operated either in a mass-flow controller mixing mode or in a chemical reaction-based mode. In the first operation mode, the chemical reaction part of the system (marked by a dashed rectangle in Figure S1) is bypassed, and the calibrated mass-flow controllers are used to mix the gases from two cylinders. For the chemical reaction-based generation mode (for labeled NH 3 ), a nitrogen cylinder was used to purge the gas mixture generated in the chemical reaction part of the system through the PA cell.
A longitudinal differential PA system was employed, similarly to our previous studies 27,53−56 (see technical details in the SI). The PA spectra of the 14 NH 3 and 15 NH 3 isotopologues were recorded with gas samples either from the gas cylinders or from the chemical reaction (using pure 15 NH 4 Cl), respectively. Measurement wavelength optimization started by recording the PA spectra of the two isotopologues and water vapor by an external cavity diode laser (ECDL, Sacher TEC 520), having an output light power of about 50 mW and its wavelength tunable between 1470 and 1590 nm. Two wavelength ranges were selected for further tests (see Discussion), from which the wavelength range of 1530.5− 1533.5 nm has been chosen for further system optimization, using a telecommunication-type fiber-coupled DFB diode laser (type: FOL15DCWD-A82-19560-A, Furukawa Inc.) operating with an emitted light power of about 45 mW. This laser has been operated in a wavelength modulated mode with an unmodulated current set to be close to 300 mA, with a small amplitude sinusoidal modulation superimposed on it. Several PA spectra of the ammonia isotopes and water vapor were recorded with laser temperature tuning. For each temperature scan, the amplitude of the laser current modulation was kept fixed but changed from scan to scan. Based on the recorded spectra, an optimum laser modulation amplitude and a set of measurement wavelengths, that are least influenced by spectral interference, have been selected. Next, the measurements were accelerated by switching from temperature to current tuning. This latter operational mode was optimized in two steps: first, laser temperatures with which all the selected wavelengths are available with current tuning were screened, and then the laser temperature that provides highest possible PA signals yielding maximum sensitivity of the isotopologue concentration measurements was selected.
The concentration measurement subroutine of the operational software of the NIR-PA system was programmed in a way that after measuring at all the selected wavelengths, it converts the measured PA signals into two quantities: PA14 and PA15 (see below) characterized by high sensitivity for 14   Analytical Chemistry pubs.acs.org/ac Article sensitivity against the other isotope as well as water vapor. The measurement sequence summarized in Table 2 was followed (see also a detailed description in the SI).

RESULTS AND DISCUSSION
The PA spectra recorded using an ECDL are shown in Figure  1. To select the appropriate measurement wavelengths, the analysis of the amplitude modulation generated ECDL spectra was executed by searching for less than 1 nm wide wavelength ranges, in which strong absorption lines of both isotopes can be found, while the absorption lines of water vapor are as weak as possible. On the other hand, perfect spectral separation among the absorption lines was not a selection criterion in this phase of system optimization yet, because wavelength modulation modifies the spectra considerably. Wavelengths below 1500 nm had to be excluded because of the presence of strong water vapor absorption lines. On the other hand, both the 1520−1523 nm and the 1530.5−1533.5 nm wavelength ranges met the primary selection criteria; therefore, spectral measurements with wavelength modulated DFB lasers were executed in these wavelength ranges, by attempting the minimization of spectral cross sensitivities via the optimization of the laser operational parameters. No suitable cross interference-free wavelengths in the former range were found, that is why the latter one had been selected for further optimization.
From the series of PA spectra of the isotopologues and water vapor, recorded by temperature tuning, the one using laser modulation amplitude of 12 mA was the least affected by spectral interferences. Indeed, these spectra contain wellseparated absorption lines of both isotopically labeled compounds, while interference from water vapor absorption lines is negligible (Figure 2). The lower x-axis of Figure 2 corresponds to the temperature of one of the lasers, while the upper x-axis (i.e., laser emission wavelength) is approximated by comparing the PA spectrum of 14 NH 3 with data from the spectral database of PNNL. 57,58 Based on this laser temperature to wavelength conversion, the selected measurement wavelengths are estimated to be the following: 1531.66, 1531.73, 1531.37, and 1531.45 nm, marked as λ 1 , λ 2 , λ 3 , and λ 4 , in Figure 2, respectively.
We note that at the beginning of the system development the possibility of using the long-wavelength end of the near infrared (e.g., the wavelength range around 2000 nm), rather than the telecommunication window, was also considered. Because of the considerably stronger absorption lines in this long-wavelength region, this is indeed very attempting, but actually there are several counterarguments as well. First, one can compare the few mW light power of a typical diode laser operating in the longer wavelength range with the ≈50 mW light power of a telecommunication diode laser. Because the generated PA signal depends on the product of the optical absorption coefficient and the light power, this product is approximately equal for the two types of lasers; that is, the disadvantage of weaker absorption lines is compensated by the much higher light power of the telecommunication diode lasers. A considerable advantage of the telecommunicationtype diode lasers is their availability in a fiber-coupled construction making them very robust and facilitating their fiber coupling for increased light power. Furthermore, they have long operational lifetime (exceeding 10 years), and they are much cheaper than their long-wavelength counterparts. Because of the narrow width (≈0.5 nm) of the wavelength range that contains all the selected measurement wavelengths, the effort to switch from temperature to current tuning was successful. The laser temperature was optimized to achieve maximum system's sensitivity. In this optimized operation mode, the software repeatedly sets the unmodulated part of the laser to 176.3, 185, 137.2, and 147.4 mA varying the measurement wavelengths among λ 1 , λ 2 , λ 3 , and λ 4 , respectively. PA signals measured at these four wavelengths by using a modulated current of 12 mA are marked in the following as PA(λ 1 ), PA(λ 2 ), PA(λ 3 ), and PA(λ 4 ), respectively. After the completion of a measurement cycle (having all four PA signals measured), the operational software of the PA system calculates the following quantities for the quantification of c( 14 NH 3 ) and c( 15 NH 3 ), respectively: 3 4 Increasing the sensitivity while simultaneously decreasing the cross sensitivity is always a primary goal of multiwavelength measurement system development. From this point of view, the definition of PA14 and PA15 as a measure of the concentration of 14 NH 3 and 15 NH 3 might be surprising at first, because Figure 2 suggests that for each isotope the subtracted signals are nearly equal, apparently resulting in reduced sensitivities. Actually this is not the case, as there is an approximately 180°degree phase difference between the subtracted PA signals; that is, PA(λ 1 ) and PA(λ 2 ) have opposite phases (as well as PA(λ 3 ) and PA(λ 4 )), and consequently, the subtractions in the definition equations of PA14 and PA15 actually increase (almost double) these signals. This phase difference is the consequence of the applied wavelength modulation, and while it actually increases the sensitivity of the system, it is also an efficient tool for decreasing cross sensitivities. Indeed, whenever an interfering component generates roughly equal PA signals at the measurement wavelengths with the same phase, this subtraction diminishes its influence. Examples for efficiently suppressed spectral interferences include tails of absorption lines of small molecules, slowly varying absorption features generated by large molecules (or aerosol particles), and background PA signal generated by light absorption on the windows or walls of the PA cell. 59 The result of the calibrations performed by mass-flow controllers and chemical reaction-based gas generation method is seen in Figure 3a,b. Rectangles in Figure 3a    Finally, as shown in Figure 4, the developed NIR-PA system is capable of following sudden concentration variations with a response time of 3.5 s. Two unique properties of the system make its response time such remarkably low. First, as it is already discussed above, because of the close proximity of the selected measurement wavelengths, current tuning can be applied, and so one measurement cycle with measuring on the four selected wavelengths can be executed within less than a second. Second, the small volume (≈10 cm 3 ) of the PA cell can be flushed through completely and rapidly, even at moderate gas flow rates. Roughly speaking, there is an inverse relationship between the gas flow rate and the response time, so whenever the gas production rate in the investigated process is slow, the flow rate can be reduced at the expense of the response time. At this point, it is worth comparing photoacoustics with one of their rivaling techniques: multipath optical absorption spectroscopy. It is true that the two methods have very similar analytical parameters and also that the same measurement wavelengths and current tuning method developed here can be applied with the latter technique, too. To achieve a similar response time, however, the large volume (≈1000 cm 3 ) of the multipath cell has to be purged at much higher rates resulting in a considerable gas consumption. In the first approximation, the response time depends on the ratio of sample volume and the flow rate, respectively. 60 At a high flow rate, the response time values of multipath (direct absorption) and PA methods will be comparable. 61

CONCLUSIONS
In this work, we have developed a DFB diode laser-based PA system for the sensitive, selective, and rapid detection of isotopically labeled NH 3 . Measurement wavelengths of the isotopologues, that are among the strongest in the targeted wavelength range and lay sufficiently close to each other to facilitate the application of current tuning of the diode lasers, were selected to program the operational software of the system in a way to complete a concentration measurement cycle in less than a second. This allows fully exploiting inherently fast response of the PA detection method even in such a demanding application. Altogether, because of its robustness, high sensitivity, low cross sensitivity, and short response time the presented system is expected to find numerous practical applications ranging from electrocatalytic N 2 conversion to biological studies.
Additional experimental details of the gas generation, wavelength selection, and the operation of the system (PDF)